The present invention relates to a process for minimizing the organic waste found in streams of aqueous products and streams of refined products in liquid-liquid extraction processes.
In a liquid-liquid extraction process, considerable process malfunctions may occur during further processing of aqueous phases separated from an extracting organic liquid medium when these aqueous phases still contain small quantities of extraction agents or decomposition products thereof in dissolved and/or finely suspended form and are concentrated by way of evaporation before they are processed further.
This applies in particular for extraction processes for instance the Purex or Thorex processes which are employed to reprocess irradiated nuclear fuel and/or breeder materials. The most customary reprocessing process at present is the Purex process which comprises a plurality of extraction cycles to recover valuable materials, such as uranium and plutonium, from the spent nuclear fuel. To better clarify the multitude of sometimes complicated process steps in such a process, it is the custom to combine several process steps to so-called cycles at the ends of which the materials, such as uranium or plutonium for example, are present in an aqueous solution. As a rule, the recovery is carried out by dissolving the fuel elements, usually in nitric acid, to form an aqueous solution containing uranium, plutonium, fission products, and/or breeder materials. The aqueous solution then is treated in the first extraction cycle in which the uranium and/or plutonium are extracted from the aqueous solution by bringing the aqueous solution into contact with an organic extraction medium. During this extraction, the fission products remain in the aqueous solution while the uranium and, if present the plutonium are simultaneously separated from the aqueous solution. The organic extraction medium generally comprises a mixture of (1) an organic extraction agent, such as an organophosphorus acid ester, for example tri-n-butyl phosphate (TBP), which serves as the active extractant and (2) a diluent, such as an aliphatic hydrocarbon (alkane). After the extraction into the organic extraction medium, the uranium and plutonium in the organic extract can be re-extracted to separate aqueous stripping solution to complete the first extraction cycle. This re-extraction can be achieved by first stripping the plutonium from the organic extract into an aqueous solution and then stripping the uranium from the organic extract into an aqueous solution.
The resulting two aqueous streams are product streams from the first extraction cycle and each of these streams can then be subjected to a second or fine extraction cycle to bring about further purification. In the second extraction cycle, each of the uranium and plutonium aqueous solutions are contacted with an organic extraction medium, which can again comprise TBP, to extract the uranium and plutonium in these solutions. Thereafter, the uranium and plutonium in the organic phases are again re-extracted into aqueous stripping solutions which comprise the fine aqueous product streams containing refined products from the second extraction cycle. An additional extraction cycle is sometimes introduced before the fine purification to produce a further purification for uranium and plutonium.
The aqueous solutions of products and refined products obtained at the end of the various extraction cycles contain small quantities of the organic extraction agent, such as TBP, and the extraction agent is in part hydrolytically decomposed during the concentration process before further processing of the solutions. With TBP, the resulting hydrolysis products are dibutyl phosphorus acid ester (HDBP), monobutyl phosphorus acid ester (H.sub.2 MBP) and H.sub.3 PO.sub.4. The resulting hydrolysis products may lead to considerable process malfunctions, e.g. clogging, increased loss of values, worsening of the fission product decontamination etc., since some corrosion and fission products form difficultly soluble precipitates with the TBP hydrolysis products and the values, mainly plutonium, are bound in a complex manner by the hydrolysis products. Without an appropriate pretreatment, the TBP carried along, and mainly its hydrolysis products HDBP and H.sub.2 MBP, would, moreover, be enriched in these solutions.
In order to avoid these malfunctions, a number of methods have been proposed to remove interfering substances from the aqueous product solutions, but each of these methods has drawbacks. In one such method used in the past, it has been the custom to proceed in the reprocessing of nuclear fuels by subjecting the aqueous process stream, for example, to a so-called kerosene wash in mixer settlers or pulsating columns before the evaporation. The kerosene here extracts the TBP from the aqueous phase. In the Purex process, the kerosene wash is presently being preferred. Only a few, usually not reproduceable results are available about the effectiveness of such a kerosene wash which is operated at extreme flow conditions (aqueous/organic(A/O) ratio equal to about 50 to 100/l) in hot radioactive operation.
The high flow conditions during the kerosene wash lead to hydraulic malfunctions in the extraction apparatuses, e.g. poor mixing of the phases, changes in the phase relationships, etc. These malfunctions considerably reduce the effectiveness of the kerosene wash and no uniform TBP separation is achieved.
In a second method proposed in the past to remove the traces of extraction agents from the aqueous phase, activated carbon is used as an adsorption agent to remove the traces of extraction agent by adsorption. Adsorption by activated carbon is described in detail by R. E. Lerch in the report of the Battelle Memorial Institute, Pacific Northwest Laboratory, Richland, Washington, Report No. BNWL-1109, "Carbon Bed Treatment of Aqueous Plant Waste for Removal of Organic Materials", August, 1969. The drawbacks of this sorption method are that even the most efficient types of activated carbon do not sorb selectively and can be regenerated only in part. Further, the activated carbon retains undesirable plutonium which must be recovered from the carbon to avoid Pu losses. The treatment with activated carbon thus does not make the Purex process simpler, but more complicated and thus more subject to malfunctions.
In a third method proposed in the past, the so-called steam stripping process is used to remove the traces of extraction agent. The steam stripping process is similar to a steam distillation in a laboratory and removes the small quantity of organic extraction agent from the aqueous solution. Steam stripping, however, requires a very large quantity of steam compared to the small quantity of TBP involved and thus requires greater amounts of apparatus, space and safety measures, etc. Moreover, a large quantity of contaminated exhaust steam or contaminated waste water is produced in this process.